An installation unit is used in machines or electrical systems, which are operated by way of command devices, such as pushbuttons, selector switches, etc., which act on the controller. These command devices are fitted in control panels, operator panels, switching cabinet doors or housing covers. Command devices are generally of modular structure; in other words they comprise an actuator, a securing element (for example a ring nut or assembly support) and one or more switching elements, which are in the form of opening or closing contacts. For assembly purposes the actuator is generally inserted from the front through a hole in the control panel and assembled from the rear using a securing element. The switching elements are connected mechanically to the actuator or securing element by means of screws, snap-fit hooks or locks. The electrical connections of the switching elements are connected electrically to the controller by way of the terminals.
In the case of safety applications, such as EMERGENCY STOP command devices, it is a requirement that the signal is generated by the opening of necessarily opening contacts. In other words in the case of a non-actuated EMERGENCY STOP command device the contacts and therefore the associated electric circuit are closed. In the event of a breakdown or emergency, hitting the EMERGENCY STOP actuator, which is located in front of the control panel, causes the opening contact to be broken and the system or machine to switch to a safe state. However this only functions, if the spatial assignment between the actuator and the switching element is ensured. As a result of incorrect assembly or the action of a force it can happen that the switching elements are separated mechanically from the actuator. The EMERGENCY STOP command device can then no longer function; in other words when actuated in an emergency the contacts are not opened and the danger state is therefore not eliminated. This can result in fatal damage to man and machine.
Since, when the switching element is spatially separated from the actuator, the opening contacts remain, closed, this error cannot be detected by the controller. The operator cannot detect this error either, since it is normally not possible to see the switching elements, which are located behind the control panel or a housing. In specific applications the switching elements are not connected in a mechanically secure manner to the actuator. This is the case, when the switching elements including wiring are assembled in the base (for example on a DIN rail) in a housing and the actuator is secured in the cover. In this instance the required spatial assignment between the actuator and the switching elements depends on the cover being properly closed. If in the case of an actuated EMERGENCY STOP device the cover of a housing or the switching cabinet door is opened, the contacts, which are opened on actuation, close and the EMERGENCY STOP command is canceled, which can have dire consequences.
DE 41 01 493 and U.S. Pat. No. 6,198,058 disclose switching elements, which try to prevent or at least to detect the errors described above. These switching elements contain an opening and closing contact. In the non-actuated state the opening contact is closed and the closing contact is open. The switching element slider is hereby configured in such a manner that when the switching elements are correctly secured at the actuator the slider is actuated to the extent that the closing contact is closed but not to the extent that the opening contact is opened. If the opening and closing contacts are connected in series and there is correct spatial assignment of the switching element and the non-actuated actuator, the electric circuit is thus closed. On actuation of the EMERGENCY STOP device the opening contact is opened and the electric circuit is broken despite the closing contact still being closed. If the switching element becomes detached as a result of incorrect assembly or the action of a force, the partially pretensioned switching element slider moves due to spring force into its initial position and the electric circuit is broken by the opening of the closing contact. Since the closing contact is opened by a spring, there is no necessary opening within the meaning of the requirement (see EN 60947-5-1).
One solution patented in JP 2003 272468 is based on a special securing mechanism. The securing elements, which are secured on a support, are secured to the actuator by means of a twist lock. This twist lock is designed so that it opens the opening contact in any position other than the locking position. If the twist lock is moved into the locking position, without the switching elements being present at the actuator, or if there is an action of a force, the opening contact remains closed, even though the spatial assignment of the switching element and the actuator is not present. It is therefore not ensured that the relevant switching element is actually ready to operate at the actuator when the EMERGENCY STOP circuit is closed. One advantage compared with the two first-mentioned solutions is that the opening contacts are necessarily opened on actuation of the twist lock from the locked to the unlocked position.
In one solution patented in JP 2003 303527 the switching element slider, which supports the movable contact piece, is connected positively to the plunger of the actuator. If this connection is not present, the opening contact is opened by a spring, which acts on the switching element slider in the actuation direction. There is no necessary opening due to the opening of the contacts by spring force. The same mode of operation is used for a solution patented in WO 2004/023501. The particular feature here is the connection of the switching element slider and the actuator plunger by means of a bayonet lock.